Gas fuelled flameless heaters rely on a flameless combustion or oxidation of a gaseous hydrocarbon fuel in order to produce heat. The hydrocarbon fuel is typically natural gas or propane. The oxidation reaction involves an exothermic reaction between the hydrocarbon fuel and oxygen, typically in the presence of a catalyst, to produce water vapor, carbon dioxide and infrared energy (i.e. heat).
A typical flameless heater includes a housing which typically contains a catalyst pad. The catalyst pad is typically located adjacent to the front of the housing. The catalyst pad includes a catalyst which is carried on a substrate to provide a catalyst bed which serves as the site of the oxidation reaction. The housing also typically contains an electrical heating element which is used to preheat the catalyst pad during start-up of the flameless heater.
The hydrocarbon fuel is delivered to the back of the flameless heater, where it is distributed so that the fuel is dispersed throughout the catalyst pad. The front of the flameless heater is covered with a screen but is otherwise open to the heater environment so that ambient air can contact the catalyst pad in order to supply the catalyst pad with the oxygen needed to react with the hydrocarbon fuel. Insulation in the form of one or more ceramic fiber pads is typically provided behind the catalyst pad so that the heat produced by the oxidation reaction is directed toward the front of the heater. The water vapor and carbon dioxide which are produced by the oxidation reaction exit the heater from the front of the catalyst pad and are typically eliminated from the heater environment with some form of venting which removes these reaction products to a location outside of the heater environment.
Flameless heaters provide several advantages in heating applications. First, the absence of a flame enables flameless heaters to be used in hazardous locations where flammable materials are handled, processed, used or stored. Second, the absence of carbon monoxide as a reaction product of the oxidation reaction enables flameless heaters to be used in indoor locations as long as proper ventilation is provided.
As a result of these advantages, flameless heaters are frequently used in oil and gas installations, which are often classified as Class I hazardous locations. For example, flameless heaters are often used to heat structures at oil and gas installations, which structures may be used to house equipment associated with wellheads, pipelines, batteries etc.
One characteristic of flameless heaters is that they produce low intensity infrared heat which is absorbed by objects within the range of the heater. The closer an object is to the heater, the more heat the object will absorb.
As a result of this characteristic, some attempts have been made to employ a flameless heater at oil and gas installations as a heat source for heating equipment which is located outside of the structure which houses the heater.
U.S. Pat. No. 6,776,227 (Beida et al) describes an apparatus and method for heating and preventing freeze-off of wellhead equipment. The apparatus includes a heat exchanger having an interior reservoir, a filler opening, a fluid outlet and a fluid inlet. The heat exchanger is preferably a finned radiator in the nature of an automotive radiator. The apparatus further includes a conduit loop running from the fluid outlet to the fluid inlet and a pump for circulating a fluid through the heat exchanger and the conduit loop. The conduit loop includes a supply section for supplying heat to wellhead equipment from heated fluid and a return section for returning cooled fluid to the heat exchanger.
The heat exchanger is positioned sufficiently close to a heat-radiating element of a flameless heater such as a gas catalytic heater such that the fluid within the interior reservoir of the heat exchanger may be heated by radiant heat from the flameless heater. The heat exchanger may be mounted onto the flameless heater with brackets or in some embodiments, the heat exchanger may be sandwiched between a pair of flameless heaters.
U.S. Pat. No. 6,776,227 (Beida et al) further describes a gas supply system for use with a heating apparatus including a heat exchanger, a gas heater and a gas-driven pump for circulating fluid from the heat exchanger, which gas supply system includes a primary gas line for delivering pressurized gas from a main gas supply for driving the pump, a secondary gas line for carrying exhaust gas from the pump to the gas heater to fuel the heater, a back-up fuel gas supply line in communication with the secondary gas line, a valve mounted in the back-up fuel gas supply line, and a valve-actuating means for opening and closing the valve. The gas supply system enables a single source of gas both to drive the gas-driven pump and fuel the gas heater.
The invention described in U.S. Pat. No. 6,776,227 (Beida et al) relies upon positioning the heat exchanger sufficiently close to the flameless heater so that radiant heat from the flameless heater provides heat to the heat exchanger. The use of radiant heat results in the apparatus being relatively inefficient, with the degree of inefficiency being directly proportional to the distance between the heat exchanger and the flameless heater.
There remains a need for a heat exchanging apparatus which utilizes a flameless heater as a heat source for a heat exchanger loop, but which is relatively more efficient than the apparatus described in U.S. Pat. No. 6,776,227 (Beida et al).